THE PLANT A LIVING MACHINE. 21 



ments in connection with this subject of absorption 

 of solar energy, where the maximum accumulation 

 of oxygen-loving bacteria at those parts of a green 

 alga which lie in the red-orange of the spectrum, 

 are used as indicators of the maximum oxygen 

 evolution (and therefore of the maximum carbon- 

 dioxide assimilation), but space will not admit of 

 this. For a similar reason I must also pass over 

 the same observer's experiments with plants which 

 assimilate in protoplasm behind a red instead of 

 a green substance, and which absorb chiefly other 

 rays between the yellow and blue, with the remark 

 that they also seem to imply that it is the proto- 

 plasmic machinery which turns the energy on to the 

 carbon-dioxide molecule, the coloured screen being 

 secondary in the matter. Recent experiments 

 which show that green plants will not assimilate 

 carbon-dioxide in a light which has passed through 

 a solution of chlorophyll and therefore left its 

 red rays behind ; nor behind a screen of iodine 

 dissolved in carbon-dioxide which lets no visible 

 rays between the red and blue pass should be 

 noticed, as showing the importance of the chloro- 

 phyll and the special rays referred to, however; and 

 I oucfht at least to mention Timiriazeff's beautiful 

 proof, published in 1890, that if, on the leaf of a 

 plant left in the dark long enough to render it free 

 of starch, a bright solar spectrum is steadily pro- 

 jected for 3-6 hours, the chlorophyll then removed 

 by alcohol and the decolorised leaf placed in iodine, 

 the image of the spectrum is reproduced by the 

 different intensities of the starch bands, blue with 



